(1) Field of the Invention
The present invention relates to a transmission system and, more particularly, to a transmission system comprising concentrated subscriber line transmission apparatuses, each accommodating a plurality of user terminals, and a relaying apparatus located between the concentrated subscriber line transmission apparatuses and a communication network.
(2) Description of Related Art
In an apartment house where a number of Internet users reside, to simplify the configuration of access lines between user terminals and the Internet network, one or more concentrated subscriber line transmission apparatuses accommodating a plurality of user terminals are located and each of the concentrated subscriber line transmission apparatuses is connected to a relaying apparatus located at a switching station by a high speed transmission line of an optical fiber.
In this case, each of the concentrated subscriber line transmission apparatuses converts upward communication frames (or packets) received from the plurality of user terminals into optical signals and multiplexes the optical signals on a same optical fiber. The signal relaying apparatus forwards the communication frames received from each concentrated subscriber line transmission apparatus to the Internet network after converting the received frames from optical signals into electric signals. The signal relaying apparatus also forwards downward communication frames received from the Internet network to the concentrated subscriber line transmission apparatuses after converting the received frames from electric signals into optical signals. Each concentrated subscriber line transmission apparatus converts the communication frames received from the signal relaying apparatus from optical signals into electric signals and forwards the received communication frame to a subscriber line specified by the destination address of the frame.
In this specification, the relaying apparatus that accommodates a plurality of concentrated subscriber line transmission apparatuses and forwards received frames or packets after performing optical-electric conversion or electric-optical conversion of the frames, as above, is referred to as a “concentrated media converter (MC)”. A high-speed concentrated subscriber line transmission apparatus to be connected to the above concentrated MC via an optical fiber line is referred to as a “concentrated VDSL (Very high bit-rate Digital Subscriber Line) apparatus.”
A control terminal is connected to the concentrated MC to monitor the status of each concentrated VDSL apparatus. An operator can transmit a control frame (or control packet) from the control terminal to each of concentrated VDSL apparatuses. In response to the control frame, each of the VDSL apparatuses generates a notification frame indicating its status and transmits the notification frame to the control terminal. In this relation, because it is disadvantageous from a cost perspective to lay additional communication lines dedicated for performing monitoring control between the concentrated MC and concentrated VDSL apparatuses, an in-channel transmission is preferably adopted to share the optical fiber as the main signal line for the purpose of communicating maintenance frames such as control frames and status notification frames between the concentrated MC and concentrated VDSL apparatuses.
In the case of adopting the in-channel transmission, transmission of maintenance frames must be controlled not to obstruct the transmission of user frames as the main signals. For this purpose, for example, according to Japanese Unexamined Patent Publication No. H6(1994)-318955, transmission/reception buffers for user frames are monitored so that maintenance frames are transmitted according to the situation of network bandwidth usage.